I believe that the decline in middle-class income is largely due to increasing automation. This will start to impact more highly skilled workers as well. In fact, any knowledge-based occupation is fair game for increasing automation and robotization. For example, it may be possible for a computer to analyze body scans. Before that, it would be quite easy for a doctor in say, India to analyze the scan at a fraction of the cost. This increasing automation will disconnect a large proportion of the population from an opportunity to earn a living.